Adaptive integrated hybrid with complex adaptation for digital subscriber line systems

ABSTRACT

An adaptive hybrid system is coupled to a loop for adjusting trans-hybrid loss. The system comprises a fixed portion comprising a first receiver transfer function block and a first hybrid transfer function block. The fixed portion is configured to receive a far-end signal and mitigate frequency dependent attenuation experienced by the far-end signal. The system also comprises a variable portion comprising a second receiver transfer function block and a second hybrid transfer function block configured to subtract a transmit echo from the received far-end signal.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority to, and the benefit of, U.S.Provisional patent application entitled, “Adaptive Integrated Hybridwith Complex Adaptation for DSL Optimized for VDSL,” having Ser. No.61/235,268, filed on Aug. 19, 2009, which is incorporated by referencein its entirety.

TECHNICAL FIELD

The present disclosure generally relates to digital subscriber linesystems and specifically to the use of an integrated adaptive hybridcircuit to improve receive signal quality by suppressing transmitterecho and noise.

BACKGROUND

In digital subscriber line (DSL) systems, the same cable is typicallyused for both transmitting and receiving signals within the network. Assuch, the receiver not only receives signals from the far end of thecable (such signals are typically attenuated significantly by losses inthe cable), but also transmits signals, which are not attenuated bycable losses. For typical DSL systems, signals are received withspectral contents at levels that can be less than −140 dBm/Hz, whilesignals may be transmitted at levels as high as −40 dBm/Hz. This impliesthat the DSL system should support a dynamic range in excess of 100 dB,which is particularly challenging in discrete multi tone (DMT) systemsas the peak-to-average power ratio (PAR) is approximately 15 dB. Thiseffectively adds another 12 dB on top of the linearity requirement. Thisincreases the dynamic range in excess of 112 dB when measured with asinusoidal input signal having a PAR of 3 dB.

Linearity and noise requirements of the receiver are generally notdriven by the system's ability to receive the signal from the far end asthis signal is small and has been significantly attenuated. Rather, therequirements are driven by the noise and echo signal coupling in fromthe transmitted signal as this signal is much larger as it has not beenattenuated by the cable. Most DSL systems rely on FDM (frequencydivision multiplexing), where the transmitted and received signals areat different frequencies, and where band-pass filtering is used toreduce echo and out of band noise. However, some systems such assymmetric DSL (SHDSL) systems and full overlap echo-cancelled asymmetricDSL (ADSL) systems share receive and transmit frequencies, therebymaking filtering impossible. Furthermore, filtering can be expensivefrom the standpoint of additional bill of material (BOM) if thefiltering is implemented with external filters. If the filtering isimplemented on-chip with integrated filters, the cost can be expensivefrom the standpoint of power and silicon area required.

SUMMARY

Briefly described, one embodiment, among others, includes an adaptivehybrid system coupled to a loop for adjusting trans-hybrid loss. Thesystem comprises a fixed portion comprising a first receiver transferfunction block and a first hybrid transfer function block, wherein thefixed portion is configured to receive a far-end signal and mitigatefrequency dependent attenuation experienced by the far-end signal. Thesystem further comprises a variable portion comprising a second receivertransfer function block and a second hybrid transfer function blockconfigured to subtract a transmit echo from the received far-end signal.

Another embodiment is a method performed in an adaptive hybrid circuitfor adjusting trans-hybrid loss on a loop. The method comprisesdetermining a reference value of an echo signal, adjusting a transferfunction of a receive filter to pass low frequency components in theecho signal, and selecting and activating a resistor array connected inparallel to an output of the adaptive hybrid circuit. The method furthercomprises adjusting a transfer function of a receive filter to pass highfrequency components in the echo signal and selecting and activating acapacitor array connected in parallel to an output of the adaptivehybrid circuit.

Another embodiment is a method for tuning an adaptive hybrid circuit toadjust trans-hybrid loss on a loop. During initialization,two-dimensional tuning is performed on components in the adaptive hybridcircuit. The two-dimensional tuning comprises selecting a resistor arrayamong two resistor arrays that minimizes a echo signal by adjustingeither a gain of a receive path or of a hybrid path and selecting acapacitor array among two capacitor arrays that minimizes a echo signalby adjusting either a gain slope and phase of a receive path or of ahybrid path. The method further comprises applying the tuned componentsin the adaptive hybrid circuit during normal operation.

Other systems, methods, features, and advantages of the presentdisclosure will be or become apparent to one with skill in the art uponexamination of the following drawings and detailed description. It isintended that all such additional systems, methods, features, andadvantages be included within this description, be within the scope ofthe present disclosure, and be protected by the accompanying claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Many aspects of the disclosure can be better understood with referenceto the following drawings. The components in the drawings are notnecessarily to scale, emphasis instead being placed upon clearlyillustrating the principles of the present disclosure. Moreover, in thedrawings, like reference numerals designate corresponding partsthroughout the several views.

FIG. 1 shows a schematic of a prior art hybrid circuit.

FIG. 2 shows the measured trans-hybrid loss for a passive hybriddesigned for a TP-100 loop.

FIG. 3 illustrates the difference in trans-hybrid loss across fourdifferent ports with schematically the same identical passive hybrid.

FIG. 4 illustrates the increase in trans-hybrid loss after implementingcapacitive tuning.

FIG. 5 depicts an embodiment of an adaptive hybrid circuit, where thehybrid and receive networks are separated into a fixed and a variableportion.

FIG. 6 depicts an embodiment of an adaptive hybrid circuit, where thefixed part comprises external RC networks and the variable partcomprises programmable resistor and capacitor arrays.

FIGS. 7-8 are flowcharts for adjusting or tuning the adaptive hybridsystems shown in FIGS. 5 and 6.

FIG. 9 illustrates a communications network in which embodiments of anadaptive hybrid system may be implemented.

FIG. 10 is a schematic block diagram of other components within anadaptive hybrid system.

DETAILED DESCRIPTION

Having summarized various aspects of the present disclosure, referencewill now be made in detail to the description of the disclosure asillustrated in the drawings. While the disclosure will be described inconnection with these drawings, there is no intent to limit it to theembodiment or embodiments disclosed herein. On the contrary, the intentis to cover all alternatives, modifications and equivalents includedwithin the spirit and scope of the disclosure as defined by the appendedclaims.

As described earlier, linearity and noise requirements of the receiverare not driven by the system's ability to receive the signal from thefar end as this signal is small and has been significantly attenuated.Rather, the requirements are driven by the noise and echo signalcoupling in from the transmitted signal as this signal is much largersince it has not been attenuated by the cable. In order to reduce theenergy of the transmit signal that feeds into the receive side, hybridcircuits are commonly utilized to subtract the transmitted signal fromthe combined transmitted and received signal observed on the cable. Theamount of rejection related to the transmitted signal achieved in hybridcircuits is measured as trans-hybrid loss. As the echo typicallydictates the linearity requirements and gain in the receive path, anyincrease in trans-hybrid loss reduces the out-of-band transmit noiserequirements and analog-to-digital converter (ADC) requirements. Forexample, 20 dB of trans-hybrid loss means that any noise and signal fromthe transmit side will be attenuated by 20 dB before it is fed into thereceiver.

Unless the gain is limited by receive signal energy (which is normallynot the case), 20 dB of gain can be added before the ADC if a 20 dBtrans-hybrid loss is achieved, thereby reducing the ADC requirement by20 dB. Accordingly, it is important to design a system which canreliably achieve high trans-hybrid loss as this allows the remainder ofthe system to be significantly simplified. With relatively hightrans-hybrid loss, the critical block in the receiver is the first gainstage in the receive chain. In this regard, as a result of poor hightrans-hybrid loss, noise in the transmit path will degrade receiverperformance, and significant noise will be added by the ADC as minimalgain can be added in front. This ultimately leads to a much morecomplicated analog front-end (AFE) design, thereby increasing powerconsumption.

To achieve high trans-hybrid loss, impedance matching is performed.Generally, this is achieved by examining a signal from the loop thatcontains the combined transmit and receive signal. Next, arepresentation of the transmitted signal is generated. Therepresentation of the transmitted signal is then subtracted from thecombined signal such that only the receive signal remains. Reference ismade to FIG. 1, which depicts a schematic of a prior art hybrid circuit100. The fixed hybrid circuit 100 shown exhibits a first-order degree ofcancellation of the transmitted signal echo and transmitter noise. Anestimate of the echo signal is derived using a voltage dividercomprising R₂ and Z_(m). The resulting signal is denoted as V_(HYB).This signal is subtracted from the original receive signal V_(RX). Byproperly selecting resistor value R₂ and impedance Z_(m), perfectcancellation of the transmitter echo and noise can be achieved. Thebalance condition is Z_(m)/(R₂+Z_(m))=Z_(L)/(R₁+Z_(L)).

To obtain at least 40 dB of trans-hybrid loss, the assumed lineimpedance and actual line impedance should ideally be within a twopercent match of each other. This, however, makes it impractical to usea single external hybrid if a single platform is to support multipleloop types, including but not limited to, twisted pair type TP-100,TP-150, and 0.4 mm cables as the characteristic impedance associatedwith these loop types varies between approximately 90 Ohms andapproximately 150 Ohms. This is significant as it is difficult to matchthis range of impedances with a single network. For higher frequencies(e.g., 30 MHz) encountered for 6-band very-high-bitrate DSL (VDSL)systems, random variations in transformer and board parasitic effectsalso makes it very difficult to maintain this level of impedancematching as a difference of just a few picofards (pF) significantlyreduces the trans-hybrid loss at these high frequencies. In this regard,it is extremely difficult to design cost and power efficient CODECs thatperform well over a variety of different line conditions andfrequencies. Thus, conventional platforms are typically optimized for aspecific band plan and line condition.

There are other more expensive, less adaptive solutions currently usedin the DSL market place. One approach is to rely on a single externalhybrid without any means of adaptation. With this approach, the sameexternal hybrid is used for different loop types (e.g., TP-100 andTP-150). However, this approach yields only about 20 dB of trans-hybridloss. The nominal impedance for a TP-100 loop is approximately 100 Ohms,and the nominal impedance for TP-150 is approximately 150 Ohms. As bothimpedances cannot be matched with a single external impedance, a 125 Ohmexternal hybrid is typically implemented to achieve 20 dB oftrans-hybrid loss. However, such low trans-hybrid loss puts a very heavyburden on the rest of the signal path as only 20 dB of rejection existsfor the transmit signal, thereby resulting in such side effects as largeecho, a large degree of transmit noise bleeding into the receive signal,and minimal gain before the ADC. The minimal gain results in asignificant contribution of the ADC noise.

Yet another conventional approach involves utilizing population optionswith different external hybrids for different loop types (e.g., TP-100,TP-150, 0.4 mm loops). One perceived shortcoming with this approach,however, is that it forces the service provider to use differentplatforms for different loops, thereby complicating the qualification,deployment, and maintenance processes. Another approach is use severalpassive, external hybrids and select the one that provides the highesttrans-hybrid loss for a given loop condition. This approach, however, isnot practical for central office (CO) platforms which tend to bedensity-driven such that external components, signal routing, and devicepin count are critical to design considerations.

Various embodiments are directed to integrated adaptive hybridsconfigured to adapt to various line impedances and compensate for randomcapacitive, parasitic effects that become critical for higher frequencyoperations required in VDSL systems. Embodiments of a single passiveexternal hybrid system are described that incorporate one characteristicline impedance (e.g., TP-100). For such embodiments, the transmit echois subtracted from the combined transmit/receive signal by providing aprogrammable relative gain between the hybrid path and the receive path.

Reference is made to FIG. 2, which shows the measured trans-hybrid lossfor a conventional passive hybrid optimized for a TP-100 loop. As shownby the first curve 210, the trans-hybrid loss is excellent, averagingabout 45 dB for all frequencies between 500 kHz and 20 MHz. The scale onthis measured data is −10 dB to +90 dB trans-hybrid loss in 10 dBincrements. However, when connecting the same external hybrid (which isoptimized to match a TP-100 cable) to a TP-150 cable or line simulator,the measured trans-hybrid loss drops sharply by approximately 25 dB, asillustrated by the second curve in 220 in FIG. 2. This is a 20+dBreduction in trans-hybrid loss, which means that the system is now tentimes more sensitive to noise coupling in from the transmitted signal.Furthermore, the system will have ten times less gain before the ADCstage, which effectively makes the ADC noise ten times more significant.

Various embodiments incorporate a two-step optimization to compensatefor such mismatches in the impedance. First, the relative gain tuning isperformed to optimize for low frequency rejection. At low frequencies,the trans-hybrid loss can be improved by approximately 12 dB (asrepresented by trace 230), which reduces the receive signal degradationimposed by the echo signal by a factor of four. However, at the highfrequencies typically utilized for short loop VDSL2 systems, more thansimple gain scaling is needed in order to achieve a good impedancematch. Therefore, the second step comprises adjusting the capacitiveimpedance separately. Through this step, an additional 12 dB oftrans-hybrid loss can be achieved (trace 240) even for frequencies up to20 MHz. Significantly, through this approach, similar performance can beachieved with a single external hybrid for both TP-100 and TP-150 loopswith a four-times reduction in the requirements for the transmit pathand the ADC in the CODEC as compared to a comparable solution using asingle comparable external passive hybrid.

Based on the approach described above for adaptive gain and capacitiveinput, exemplary embodiments of an adaptive hybrid system may also befine tuned to address random mismatches and capacitive parasitics. Suchrandom mismatches and parasitic effects can be attributed to variousfactors. For example, the external routing is typically differentbetween different channels. Furthermore, the actual leakage inductancecan vary between various transformers on a given board. By utilizingadaptive capacitive tuning in accordance with various embodiments,calibration can be performed to mitigate capacitive mismatches byadaptively adding capacitance as needed.

To illustrate the variations in trans-hybrid loss, reference is made toFIG. 3, which shows the trans-hybrid loss measured for four differentports using schematically the same identical passive hybrid. As shown,there is almost a 10 dB difference in actual measured trans-hybrid lossacross the four ports at 12 MHz. The actual trans-hybrid loss measuredvaries between 40 dB and 50 dB for the four ports. The differences intrans-hybrid loss between the ports can be attributed to variousfactors, including but not limited to board routing, differences inactual values for the discrete components used to implement the externalhybrid, and differences in leakage inductance between the fourtransformers.

As one skilled in the art will appreciate, hybrid matching becomesparticularly challenging at high frequencies as the system becomessusceptible to parasitic effects and capacitive differences at thosefrequencies. To address perceived shortcomings with conventional hybridcircuits, various embodiments incorporate an adaptive capacitive schemeusing internal adaptation to compensate for differences introduced byparasitic effects and differences in the external hybrid to achieve ahigh degree of impedance matching. Reference is made to FIG. 4, whichshows results obtained prior to using capacitive tuning where the worsttrans-hybrid loss measurement increased from 40 dB back to 45 dB. Whilethe initial offset and the required correction are generally small, itshould be emphasized that the advantages of capacitive tuning willbecome much more significant across a larger population of ports inminimizing port-to-port performance differences.

Exemplary embodiments of a hybrid system significantly improvetrans-hybrid loss by using a single, common external hybrid andadaptively calibrating the integrated hybrid function for differentloops. For example, with the adaptive hybrid system described herein, 45dB of trans-hybrid loss can be achieved for a TP-100 loop, while for aTP-150 loop, the same adaptive hybrid system may be utilized to achieveapproximately 40 dB of trans-hybrid loss. Various embodiments comprisean adaptive relative gain module, which is used during subtraction ofthe transmit signal from the combined receive/transmit signal. Theadaptive relative gain module is also used to provide an adaptivecapacitive load to the hybrid input with minimal cost from thestandpoint of silicon and power consumption. Note that improving thetrans-hybrid loss by 12 dB eases the requirements on the transmit pathand ADC by a factor of four by reducing the echo level. Accordingly, thehybrid system described may be utilized across different loops without asignificant sacrifice in system performance. Other conventional,non-adaptive solutions require more stringent performance requirementson the CODEC, thereby increasing cost and/or power. Another perceivedshortcoming with conventional hybrids is that different external hybridsare needed for the different loops in order to achieve similartrans-hybrid loss. This results in an increase in the bill of materials(BOM) and ultimately, cost of the overall system.

An adaptive hybrid system and tuning method are now described, whichincorporate a calibration process to mitigate capacitivemismatches/parasitic effects in an external hybrid, thereby reducing theport-to-port variation in trans-hybrid loss. This is particularlycritical for high bandwidth VDSL applications. Some embodiments of theadaptive hybrid system are designed such that on-chip resistor-capacitor(RC) circuits are only connected when the loop deviates from the normalloop impedance. This ensures that the RC circuits do not add anydistortion or noise relative to a conventional hybrid circuit andtherefore results in high linearity and low noise. When the loopimpedance deviates from the normal impedance, the adaptive hybrid systemis configured to connect and tune the RC circuits. In accordance withsome embodiments, the tuning process may be controlled by firmware.While noise and distortion are slightly increased, the gain inperformance due to improved trans-hybrid loss significantly outweighsthe loss due to the increase of noise and distortion due to the added RCcircuits.

Reference is now made to FIG. 5, which depicts an embodiment of anadaptive hybrid system 500, where the system 500 comprises a fixedcomponent 502 and a variable component 504. The fixed component 502comprises a first receive transfer function block H_(rx1) (s) and afirst hybrid transfer function block H_(hyb1)(s). Both transfer functionblocks are used to configure the overall transfer function of theexternal component 502. Similarly, the fixed component 502 comprises asecond receive transfer function block H_(rx2)(s) and a second hybridtransfer function block H_(hyb2)(s), both of which control the transferfunction of components integrated into the analog front end device. Thesignal V_(RX) denotes the receive signal prior to echo cancellation. Thesignal V_(HYB) is the estimated echo signal that must be subtracted fromthe receive signal to cancel the echo. Signal V_(RCV) represents thereceive signal after echo cancellation, V_(RCV)=V_(RX)−V_(HYB). Perfectecho cancellation results whenH_(hyb1)(s)·H_(hyb2)(s)=H_(rx1)(s)·H_(rx2)(s)·Z_(L)/(R₁+Z_(L)).

Based on the transfer function blocks H_(rx1)(s) and H_(rx2)(s), theadaptive hybrid system 500 performs spectral shaping of the far-endreceive signal or echo. Note that such spectral shaping is separate fromthe hybrid function. The overall receive transfer function can beconfigured to be a high-pass function, a low-pass function, or aband-pass function, which allows the adaptive hybrid system 500 toattenuate unwanted signal components. Note that the adaptive hybridsystem 500 may be configured to emphasize specific signal components,e.g., high frequency components that are considerably more attenuated bythe loop than lower frequencies. When spectral shaping is not needed,transfer function H_(rx1)(s) can be set to 1. For the nominal loopimpedance, Z_(L0), the transfer functions can be set such thatH_(rx2)(s)=H_(hyb2)(s)=1. Note that the hybrid is balanced whenH_(hyb1)(s)=Z_(L0)/(R₁+Z_(L0)).

By configuring the transfer function blocks H_(hyb2)(s) and H_(rx2)(s)to 1 for the nominal impedance, the adaptive hybrid system 500 ensuresthat no noise, distortion, or attenuation is added for loops thatperform well with a standard fixed hybrid. The added transfer functionblocks H_(hyb2)(s) and H_(rx2)(s) are only activated for loops with adeviating impedance (when system performance is significantly impactedwith a fixed hybrid). When the loop impedance deviates from the nominalimpedance Z_(L0), the adaptive hybrid system 500 activates either blocksH_(hyb2)(s) or H_(rx2)(s), and the corresponding transfer function isadjusted to restore hybrid balance. While it would be sufficient toprovide only H_(hyb2)(s) for this purpose and leave H_(rx2)(s) out, theinclusion of transfer function block H_(rx2)(s) can result in simplerimplementation overall.

FIG. 6 represents one possible implementation of the adaptive hybridsystem 500 in FIG. 5. The transfer function blocks (H_(rx1)(s) andH_(hyb1)(s)) in the variable component of the adaptive hybrid system 500comprise two external resistor-capacitor (RC) networks for configuringtransfer functions H_(rx1)(s) and H_(hyb1)(s). The transfer functionsblocks H_(rx2)(s) and H_(hyb2)(s) are implemented in an analog frontend. The analog front end further comprises capacitor arrays (C4, C6),resistor arrays (R4, R7), and resistor R8. For this implementation, thecapacitor and resistor arrays are programmable and can be disconnectedfrom the signal path when not in use. Furthermore, for someimplementations, the capacitance and/or resistance values can be set bywriting into registers through firmware.

Note that for the implementation shown in FIG. 6, the adaptive hybridsystem 500 does not include buffer amplifiers between the external andinternal portions of the hybrid system 500. In the hybrid system 500, itis implicitly assumed that the internal and external transfer functionsare independent of each other. For example, the transfer functionH_(hyb2)(s) is assumed to be independent of H_(hyb1)(s). Therefore,H_(hyb1)(s) does not change when H_(hyb2)(s) is adjusted to balance thehybrid system 500 and vice versa. Independence between blocks may beachieved by placing a buffer amplifier between blocks. In practice, itis not necessary for the two transfer functions to be completelyindependent as long as H_(hyb1)(s) does not change substantially whenH_(hyb2)(s) is adjusted. This is generally true when the loadingpresented by H_(hyb2)(s) on H_(hyb1)(s) is relatively small and appliesto the implementation shown in FIG. 6, since generally, the values ofthe resistor arrays R4 and R7 are much larger than R8 as well as R1, R2,R3, and R6. Conversely, the values of the capacitor arrays C4 and C6 aremuch smaller than C2, C3, and C5. Note that by avoiding buffers in theimplementation shown, noise and distortion can be avoided. Furthermore,the device area and power consumption requirements are reduced.

In accordance with various embodiments, the adaptive hybrid system 500is calibrated based on the assumption that the adaptive hybrid system500 is connected to a loop with nominal impedance (Z_(L0)). Duringinitialization, the capacitor arrays C4 and C5 and resistor arrays R4and R7 are disconnected from the signal path (i.e., C4=C6=0, R4=R7=∞).The receive transfer function H_(rx1)(s) has a high-pass characteristicdetermined by the values of capacitor C6 and resistors R6, R8, and R9.Components R1, R2, R3, C1, C2, and C3 create transfer functionH_(Hyb1)(s) that balances the hybrid. While it is possible to derive thecomponent values in closed form for special cases of Z_(L0), a numericaloptimization process is generally utilized to maximize the trans-hybridloss over a frequency range of interest.

If the loop impedance of the adaptive hybrid system 500 is differentfrom Z_(L0), resistor arrays R4 and R7 and capacitor arrays C4 and C6are connected, and their values are tuned to balance the hybrid. Notethat the values of R4 and R7 affect the gain across all frequencies,while C4 and C6 selectively affect the gain and phase at highfrequencies. When the actual loop impedance of adaptive hybrid system500 is lower than the nominal loop impedance Z_(L0), the signal at nodeA as shown in FIGS. 5, 6 is decreased, while the signal at node B isunchanged. In such cases, the hybrid 500 is unbalanced. Balance isrestored by reducing the gain from node B to the hybrid output 610. Thisis accomplished by connecting resistor array R4 to the signal path andselecting an optimum resistor value. More details on how optimumresistor values are derived are described later.

Conversely, when the actual loop impedance of the adaptive hybrid system500 is higher than the nominal loop impedance Z_(L0), the signal at nodeA is increased, while the signal at node B is unchanged. Again, in suchcases, the hybrid is unbalanced. Balance is restored by reducing thegain from node A to the hybrid output. This is accomplished byconnecting resistor array R7 to the signal path and selecting an optimumresistor value. Capacitor arrays C4 and C6 are similarly used, but theircapacitance only affects gain and phase at high frequencies. Forexample, suppose that excess capacitance is present at the lineinterface in parallel to the loop. The capacitance may be the inputcapacitance of a service splitter which combines the xDSL signal with aconventional telephony signal towards the subscriber line. Because ofthe excess capacitance, the signal at node A (in FIGS. 5, 6) rolls offtowards high frequencies relative to the nominal case, while the signalat node B remains unchanged. Therefore, the hybrid is unbalanced.Balance is restored to a first order of approximation by creating anequivalent roll-off in the signal path between node B and the hybridoutput. This is accomplished by connecting capacitor array C4 to thesignal path and selecting the correct capacitance.

In other instances, excess inductance may be present at the lineinterface in series with the loop. The inductance may be caused forexample, by a line transformer with high leakage inductance. Because ofthe excess inductance, the signal at node A increases towards highfrequencies relative to the nominal case, while the signal at node Bremains unchanged. In this case, the hybrid is unbalanced. Balance isrestored to a first order of approximation by connecting the capacitorarray C6 to the signal path between node A and the hybrid output andselecting an optimum capacitance value.

In the cases described above, the amount of capacitance added is quitesmall (generally less than a few tens of picofarads) and therefore, forsome embodiments of the adaptive hybrid system 500, variable capacitance(via capacitor arrays) is implemented inside the analog front enddevice. As previously noted for the embodiment shown in FIG. 6, in somecases the external and internal transfer functions may not be entirelyindependent. For example, the cutoff frequency and gain of the high-passreceive transfer function H_(rx1)(s) changes slightly when the capacitorarray C6 or the resistor array R7 is connected to the signal path.However, since the resistor value of R7 is generally relatively large(on the order of several kilo-ohms) and the capacitance of C6 isrelatively small (on the order of a few picofarads) compared to C5, R6and R8, the impact is relatively small.

Having described the basic framework for an adaptive hybrid system 500,a selection process for deriving optimum resistor and capacitor valuesof the resistor and capacitor arrays in FIG. 6 is now described. Inaccordance with various embodiments, optimum resistor or capacitorvalues for the component arrays shown are determined iteratively throughan optimization process, which may be implemented in firmware. Referenceis made to FIG. 7, which is a flow chart 700 of an optimization processfor automatically tuning the adaptive hybrid system 500. Beginning withblock 710, the adaptive hybrid system 500 is initialized to defaultsettings. In block 720, the transmitter signal and transmit filters areconfigured to generate an optimum hybrid response. These settings willbe identical or similar to those used during normal operation of thetransceiver, but it is possible to emphasize certain frequency bands.The settings are adjusted to generate a particular response, which maycomprise, for example, a particular bandwidth response, output level,spectral shape, and so on. The settings are also selected so that thenormal xDSL handshaking and startup processes of the overall system arenot affected.

Similarly, in block 730, the receive filters and gain are alsoconfigured. As described earlier, the overall receive transfer functioncan be configured to be a high-pass function, a low-pass function, or aband-pass function, which allows the adaptive hybrid system 500 toattenuate unwanted signal components. In block 740, the transmit signalis applied and the echo spectrum is measured at the output of thereceive path (block 750). In block 760, the parameters of the adaptivehybrid are iteratively adjusted such that the echo power is minimizedand/or to achieve a specific desired response. These optimum parametervalues are stored for use during the normal startup process. Thecomplexity of the optimization algorithm varies depending on the numberof parameters.

For the implementation depicted in FIG. 6, four parameters may beincorporated. Note, however, that for some embodiments, the complexityof the optimization process can be further reduced by only adjustingjust two parameters at a time. In particular, only one resistor arrayand one capacitor array will be active at any given time, while theother arrays remain disconnected. With this configuration, the followingtwo dimensional tuning process is performed. With reference to FIG. 8,in block 810, the low frequency trans-hybrid loss is optimized byselecting a transfer function for the receive filter that emphasizes lowfrequency components of the echo signal. In block 820, the initial valueof echo power is determined. In block 830, a determination is made onwhether the echo can be reduced by connecting resistor array R5 or R7.Resistor arrays and capacitor arrays are initially disconnected. Tuningof the resistor array is performed by switching in the highest possiblevalue of one of the resistor arrays. The value of the array is thendecreased step by step until an optimum value is found. Conversely, thecapacitor array is tuned by switching in the smallest (non-zero)capacitor value possible. The value of the capacitor array is thenincreased until an optimum value is found. Once the optimumconfiguration has been determined (e.g., whether to connect resistorarray R5) for maximizing trans-hybrid loss, the resistor value thatminimizes echo is determined based on an iterative process (block 840).During this process, the second resistor array (i.e., R7 in theimplementation shown earlier) is disconnected.

In block 850, the high frequency components of the echo signal areemphasized by selecting a receive filter that boost high frequencycomponents of the echo signal. In block 860, the initial echo responseis determined. In block 870, a determination is made on whether the echocan be reduced by connecting capacitor arrays C4 or C6. Once the correctcapacitor array has been determined (e.g., whether to connect capacitorarray C6), the capacitor value that minimizes echo is determined basedon an iterative process. During this process, the second capacitor array(i.e., C7 in the implementation shown earlier) is disconnected (block880). In block 890, the optimum settings derived for both the lowfrequency and high frequency components of the echo signal are thenstored for later use during normal operation. The two dimensional tuningprocess outlined above relies on the fact that the resistor arraysaffect overall gain and therefore the low frequency behavior of theadaptive hybrid system 500, while the capacitor arrays mainly affect thegain slope and phase at high frequencies and therefore the highfrequency behavior of the hybrid.

It should be emphasized that the processes outlined in the flowchartsabove may be in embodied in software, hardware, or a combination of bothsoftware and hardware. If embodied in software, each block depicted inFIGS. 7 and 8 represents a module, segment, or portion of code thatcomprises program instructions to implement the specified logicalfunction(s). In this regard, the program instructions may be embodied inthe form of source code that comprises statements written in aprogramming language or machine code that comprises numericalinstructions recognizable by a suitable execution system such as aprocessor in a communication system or other system such as the oneshown in FIG. 9. The machine code may be converted from the source code,etc. If embodied in hardware, each block may represent a circuit or anumber of interconnected circuits to implement the specified logicalfunction(s). Furthermore, as one of ordinary skill in the art willappreciate, other sequences of steps may be possible, and the particularorder of steps set forth herein should not be construed as limitationson the claims.

The processes described provides a fast and efficient means (for someimplementations, on the order of milliseconds) to complete eachoptimization step and on the order of a few tenths of a second tocomplete the overall process. The processes described can be expanded toincorporate joint optimization of the values of the resistor andcapacitor arrays. The processes can also be configured to analyze thespectral content of the echo signal (e.g., via an FFT module) instead ofoverall power and use this information to build a complex goal functionthat drives the optimization process.

Reference is now made to FIG. 9, which illustrates a communicationsystem 900 in which the described embodiments of an adaptive hybridsystem may be implemented. In accordance with some embodiments, thecommunication system 900 may comprise a DMT-based xDSL system. Asdepicted in FIG. 9, the communication system 900 includes a centraloffice (CO) 930 and a plurality of CPE (customer premises equipment)devices 910 a, 910 b, 910 c, where each device 910 a, 910 b, 910 c isreferenced by index n. The CO 130 includes an xDSL access multiplexer(DSLAM), xDSL line cards 140 a, 140 b, 140 c, and other equipment forinterfacing with users 910 a, 910 b, 910 c. In some embodiments, theadaptive hybrid system 500 described herein may be implemented on the2-wire interface of each of the xDSL line cards 940 a, 940 b, 940 c. Itshould be emphasized that while embodiments of the adaptive hybridsystem 500 are described in the context of CO-centric implementations,the adaptive hybrid system 500 may also be implemented on the CPE side.

FIG. 10 illustrates an embodiment of an adaptive hybrid system 500located within the CO 930 in FIG. 9 for executing and controlling thevarious components. As described earlier, for some embodiments, thetuning process described with respect to FIGS. 7 and 8 may be controlledby firmware. Generally speaking, the adaptive hybrid system 500 maycomprise any one of a number of computing devices. Irrespective of itsspecific arrangement, the adaptive hybrid system 500 may comprise memory1012, a processor 1002, and mass storage 1026, wherein each of thesedevices are connected across a data bus 1010.

The processor 1002 may include any custom made or commercially availableprocessor, a central processing unit (CPU) or an auxiliary processoramong several processors associated with the adaptive hybrid system 500,a semiconductor based microprocessor (in the form of a microchip), oneor more application specific integrated circuits (ASICs), a plurality ofsuitably configured digital logic gates, and other well known electricalconfigurations comprising discrete elements both individually and invarious combinations to coordinate the overall operation of thecomputing system.

The memory 1012 can include any one or a combination of volatile memoryelements (e.g., random-access memory (RAM, such as DRAM, and SRAM,etc.)) and nonvolatile memory elements (e.g., ROM, hard drive, CDROM,etc.). The memory 1012 typically comprises a native operating system1014, one or more native applications, emulation systems, or emulatedapplications for any of a variety of operating systems and/or emulatedhardware platforms, emulated operating systems, etc. For example, theapplications may include application specific software 1016 stored on acomputer readable medium for execution by the processor 1002 and mayinclude applications for performing the processes outlined in FIGS. 7and 8. As discussed earlier, for some embodiments, the tuning processdescribed can be controlled in firmware. With reference to FIG. 10, theadaptive hybrid system 500 may comprise application specific software1016 configured to control specific registers associated with theadjustable components shown in FIG. 6.

Where any of the components described above comprises software or code,the same can be embodied in any computer-readable medium for use by orin connection with an instruction execution system such as, for example,a processor in a computer system or other system. In the context of thepresent disclosure, a computer-readable medium can be any tangiblemedium that can contain, store, or maintain the software or code for useby or in connection with an instruction execution system. For example, acomputer-readable medium may store one or more programs for execution bythe processing device 1002 described above. The computer readable mediumcan be, for example, but not limited to, an electronic, magnetic,optical, electromagnetic, infrared, or semiconductor system, apparatus,or device.

More specific examples of the computer-readable medium may include anelectrical connection having one or more wires, a portable computerdiskette, a random access memory (RAM), a read-only memory (ROM), anerasable programmable read-only memory (EPROM, EEPROM, or Flash memory),and a portable compact disc read-only memory (CDROM). As shown in FIG.10, the adaptive hybrid system 500 may further comprise mass storage1026. For some embodiments, the mass storage 1026 may include a database1028 for storing optimum settings, as described with reference to FIG.8.

A system component and/or module embodied as software may also beconstrued as a source program, executable program (object code), script,or any other entity comprising a set of instructions to be performed.When constructed as a source program, the program is translated via acompiler, assembler, interpreter, or the like, which may or may not beincluded within the memory component 1184, so as to operate properly inconnection with the operating system 1190. When the adaptive hybridsystem 500 is in operation, the processor 1002 may be configured toexecute software stored within the memory component 1012, communicatedata to and from the memory component 1012, and generally controloperations of the adaptive hybrid system 500 pursuant to the software.Software in memory, in whole or in part, may be read by the processor1002, perhaps buffered and then executed.

It should be emphasized that the above-described embodiments are merelyexamples of possible implementations. Many variations and modificationsmay be made to the above-described embodiments without departing fromthe principles of the present disclosure. All such modifications andvariations are intended to be included herein within the scope of thisdisclosure and protected by the following claims.

1. An adaptive hybrid system coupled to a loop for adjustingtrans-hybrid loss, the system comprising: a fixed component comprising afirst receiver transfer function block and a first hybrid transferfunction block, wherein the fixed portion is configured to receive afar-end signal and mitigate frequency dependent attenuation experiencedby the far-end signal; and a variable component comprising a secondreceiver transfer function block and a second hybrid transfer functionblock configured to remove transmit echo from the received far-endsignal.
 2. The system of claim 1, wherein the second receiver transferfunction block and the second hybrid transfer function block compriseprogrammable resistors and programmable capacitors.
 3. The system ofclaim 2, wherein resistor values and the capacitor values associatedwith the programmable resistors and programmable capacitors areadjustable via firmware by accessing registers associated with each ofthe components.
 4. The system of claim 2, wherein the programmableresistors and programmable capacitors comprise one or more programmablecapacitor arrays, one or more programmable resistor arrays, and aprogrammable resistor.
 5. The system of claim 4, wherein theprogrammable resistor arrays and the programmable capacitor arrays arecoupled in parallel with respect to the output of the adaptive hybridsystem.
 6. The system of claim 1, wherein the transfer function blocksin the fixed component are only activated if an impedance of the loopdeviates from an expected impedance by a predetermined amount, whereinthe transfer function blocks in the fixed component are configured toprovide a unity gain if the impedance of the loop does not deviate fromthe expected impedance.
 7. The system of claim 1, wherein the variableportion is integrated into an analog front end (AFE) circuit.
 8. Thesystem of claim 1, wherein variable portion performs two-dimensionaladaptive tuning to match an impedance of the adaptive hybrid system toan impedance of the loop.
 9. The system of claim 1, wherein the firstreceive transfer function block in the fixed component and the secondreceive transfer function block in the variable component are configuredto perform spectral shaping of the received far-end signal.
 10. Thesystem of claim 9, wherein the spectral shaping of the received far-endsignal comprises performing one of: low-pass filtering, high-passfiltering, and band-pass filtering on the received far-end signal.
 11. Amethod performed in an adaptive hybrid circuit for adjustingtrans-hybrid loss on a loop, comprising: receiving an echo signal;adjusting a transfer function of a receive filter to pass low frequencycomponents in the received echo signal; selecting and activating aresistor array connected in parallel to an output of the adaptive hybridcircuit; adjusting a transfer function of a receive filter to pass highfrequency components in the received echo signal; and selecting andactivating a capacitor array connected in parallel to an output of theadaptive hybrid circuit.
 12. The method of claim 11, wherein selectingand activating a resistor array comprises selecting a resistor arrayamong a plurality of resistor arrays that most effectively reduces alevel of the echo signal.
 13. The method of claim 12, further comprisingiteratively adjusting the selected resistor array to further reduce thelevel of the echo signal.
 14. The method of claim 11, wherein selectingand activating a capacitor array comprises selecting a capacitor arrayamong a plurality of capacitor arrays that most effectively reduces alevel of the echo signal.
 15. The method of claim 14, further comprisingiteratively adjusting the selected capacitor array to further reduce thelevel of the echo signal.
 16. A method for tuning an adaptive hybridcircuit to adjust trans-hybrid loss on a loop, comprising: duringinitialization, performing two-dimensional tuning of components in theadaptive hybrid circuit, wherein the two-dimensional tuning comprises:selecting a resistor array among two resistor arrays that minimizes again of the echo signal; and selecting a capacitor array among twocapacitor arrays that minimizes a gain slope and a phase of the echosignal; and applying the tuned components in the adaptive hybrid circuitduring normal operation.
 17. The method of claim 16, further comprisingdisconnecting the non-selected resistor array and capacitor array. 18.The method of claim 17, further comprising iteratively adjusting theselected resistor array to further reduce the gain of one of a receivepath and a hybrid path within the adaptive hybrid circuit.
 19. Themethod of claim 17, further comprising iteratively adjusting theselected capacitor array to further reduce the gain slope and phase ofone of a receive path and a hybrid path within the adaptive hybridcircuit.
 20. The method of claim 17, wherein the selection of a resistorarray and the selection of a capacitor array are performed jointly.